Optical fiber bundles to be used for transmission of light each include an incidence end formed by a fusion integration of ends of a plurality of optical fibers constituting the optical fiber bundle (refer to the patent document 1, for instance).
Optical fibers constituting such an optical fiber bundle typically have a core made of pure silica glass, and a cladding made of fluoridated silica glass with a smaller refractive index than the core.
Such the optical fibers employing pure silica glass for their cores have small light acceptance angles (numerical apertures) due to limitations on the fabrication method, so they are adapted to transmit no more than rays of light having income at angles within a range of 11 degrees to 14 degrees or near. As a result, for transmission of light radiated from a light source having a greater spread angle than an acceptance angle of optical fibers, the optical fiber bundle had a reduced coupling coefficient with the light source, as an issue.
On the other hand, as a recent trend in industrial fields, there is use of an ultraviolet curing method as a method of curing, among others, adhesives or coating agents. The ultraviolet curing method is a technique of irradiating an ultraviolet curable material with ultraviolet rays for causing a photopolymerization reaction to change a monomer (liquid) to a polymer (solid).
For the ultraviolet curing method, an ultraviolet irradiator provided with an ultraviolet lamp as a light source has been employed since ever. However, since development of an LED (Light Emitting Diode) adapted to emit ultraviolet rays, there have been practical applications of such an ultraviolet irradiator that uses an ultraviolet LED in place of ultraviolet lamp.
As an ultraviolet irradiator using an ultraviolet LED as a light source, there has been one disclosed in the patent document 2, for instance.
The ultraviolet irradiator disclosed in the patent document 2 has an ultraviolet LED built in an optical irradiation head for projecting ultraviolet rays. Such being the case, placing a light source acting as a heat source as well in an optical irradiation head has constituted a difficulty for the optical irradiation head to be cooled. Moreover, there have been anxieties about raised temperatures of the optical irradiation head, such as by, among others, heat liberation of irradiated objects, and body temperatures of an operator holding the optical irradiation head in the hand during operation.
And, by the rise of temperature of the optical irradiation head, the ultraviolet LED would have accumulated heat, with reduced output power, as an issue.
Patent document 1: Japanese Patent Application Laid-Open Publication No. 2006-72025
Patent document 2: Japanese Patent Application Laid-Open Publication No. 2006-281130
The present invention has been devised in view of the foregoing, so it is an object of the present invention to provide an optical fiber bundle allowing for an enhanced coupling efficiency with a light source.
It also is an object of the present invention to provide an optical irradiator using the above-noted optical fiber bundle and adapted to control light output power against variations due to heat accumulation at a light source.